The presence, nature, and impact of chemical short-range order in the multi-principal element alloy CrCoNi are all topics of current interest and debate. First-principles calculations reveal that its ...origins are fundamentally magnetic, involving repulsion between like-spin Co-Cr and Cr-Cr pairs that is complemented by the formation of a magnetically aligned sublattice of second-nearest-neighbor Cr atoms. Ordering models following these principles are found to predict otherwise anomalous experimental measurements concerning both magnetization and atomic volumes across a range of compositions. In addition to demonstrating the impact of magnetic interactions and resulting chemical rearrangement, the possible explanation of experiments would imply that short-range order of this type is far more prevalent than previously realized.
Local chemical ordering (LCO) in the CrCoNi medium-entropy alloy was investigated by transmission electron microscopy (TEM) after different annealing treatments and their corresponding mechanical ...properties by bulk tensile tests and nanoindentation. A cold-rolled alloy was annealed at 1000°C for 0.5 h followed by ice water quenching and then aged at a number of different temperatures (600°C, 700°C, 800°C, 900°C, and 1000°C) under vacuum for 240 h to generate different degrees of chemical ordering. A splat-quenched sample rapidly cooled from the liquid phase was also examined. While bulk mechanical properties did not vary among samples with equivalent grain sizes, nanoindentation tests revealed notable differences. As indicated by the load at first pop-in using a Berkovich tip or the indentation yield strength via continuous stiffness measurements using a 10 μm spherical tip, the nanoindentation tests revealed that the stress for onset of plasticity during indentation varied with heat treatment and peaked in the 900°C aged sample. Energy-filtered TEM characterization indicated the presence of ordering in all specimens, with a higher degree of LCO in the aged samples relative to the splat-quenched and 1000°C-quenched samples. The evolution of LCO during aging was determined to occur on the time scale similar to those of bulk diffusion. The difference in nanoindentation strength was attributed to the difference in dislocation nucleation barriers imposed by different degrees of LCO.
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Poor bone quality contributes to bone fragility in diabetes, aging, and osteogenesis imperfecta. However, the mechanisms controlling bone quality are not well understood, contributing to the current ...lack of strategies to diagnose or treat bone quality deficits. Transforming growth factor beta (TGF-β) signaling is a crucial mechanism known to regulate the material quality of bone, but its cellular target in this regulation is unknown. Studies showing that osteocytes directly remodel their perilacunar/canalicular matrix led us to hypothesize that TGF-β controls bone quality through perilacunar/canalicular remodeling (PLR). Using inhibitors and mice with an osteocyte-intrinsic defect in TGF-β signaling (TβRIIocy−/−), we show that TGF-β regulates PLR in a cell-intrinsic manner to control bone quality. Altogether, this study emphasizes that osteocytes are key in executing the biological control of bone quality through PLR, thereby highlighting the fundamental role of osteocyte-mediated PLR in bone homeostasis and fragility.
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•TGF-β is an osteocyte-intrinsic regulator of perilacunar/canalicular remodeling (PLR)•Osteocytes actively maintain bone quality through regulated control of PLR•Osteocytic PLR is the cellular mechanism by which TGF-β controls bone quality•Defects in PLR cause severe bone fragility, even when bone mass is normal
Resistance to fracture requires healthy bone mass and quality. However, the cellular mechanisms regulating bone quality are unclear. Dole et al. show that osteocyte-intrinsic TGF-β signaling maintains bone quality through perilacunar/canalicular remodeling. Thus, osteocytes mediate perilacunar/canalicular remodeling and osteoclast-directed remodeling to cooperatively maintain bone quality and mass and prevent fragility.
Abstract
Classical Monte Carlo simulation of the Heisenberg model poorly describes many thermodynamic phenomena due to its neglect of the quantum nature of spins. Alternatively, we discuss how to ...semiclassically approach the quantum problem and demonstrate a simple method for introducing a locally approximate form of spin quantization. While the procedure underestimates magnetic short-range order, our results suggest a simple correction for recovering realistic spin–spin correlations above the critical temperature. Moreover, ensemble fluctuations are found to provide reasonably accurate thermodynamics, largely reproducing quantum mechanically calculated heat capacities and experimental magnetometry for ferromagnetic Fe and antiferromagnetic RbMnF
3
. Extensions of the method are proposed to address remaining inaccuracies.
The seemingly contradictory state of research on short-range order in many-component alloys is addressed through a critical review of the characterization of face-centered-cubic 3
d
systems. Despite ...the paucity of direct observations, the ordering of many widely studied alloys is argued to be primarily interesting for its potential ubiquity. To clarify this situation, future research directions are proposed with reference to historical results, including a review of the fundamental principles of ordering and clustering.
Graphical abstract
The osseous sword of a swordfish (Xiphias gladius) is specialized to incapacitate prey with stunning blows. Considering the sword's growth and maturation pattern, aging from the sword's base to the ...tip, while missing a mechanosensitive osteocytic network, an in‐depth understanding of its mechanical properties and bone quality is lacking. Microstructural, compositional, and nanomechanical characteristics of the bone along the sword are investigated to reveal structural mechanisms accounting for its exceptional mechanical competence. The degree of mineralization, homogeneity, and particle size increase from the base toward the tip, reflecting aging along its length. Fracture experiments reveal that crack‐growth toughness vastly decreases at the highly and homogeneously mineralized tip, suggesting the importance of aging effects. Initiation toughness, however, is unchanged suggesting that aging effects on this hierarchical level are counteracted by constant mineral/fibril interaction. In conclusion, the sword of the swordfish provides an excellent model reflecting base‐to‐tip‐wise aging of bone, as indicated by increasing mineralization and decreasing crack‐growth toughness toward the tip. The hierarchical, structural, and compositional changes along the sword reflect peculiar prerequisites needed for resisting high mechanical loads. Further studies on advanced teleosts bone tissue may help to unravel structure–function relationships of heavily loaded skeletons lacking mechanosensing cells.
The osseous sword of the swordfish reveals a lengthwise aging pattern reflected by mineralization and toughness indices. The mineralization (low at base, high at tip) is linked to specific mechanical competence of the bone material, lacking mechanosensitive osteocytes known to initiate repair processes in humans. Thus, the swordfish presents an intriguing model to study anosteocytic mechanisms of bone quality maintenance.
Using a bidirectional freezing technique, combined with uniaxial pressing and in situ polymerization, “nacre‐mimetic” hydroxyapatite/poly(methyl methacrylate) (PMMA) composites are developed by ...processing large‐scale aligned lamellar ceramic scaffolds. Structural and mechanical characterization shows “brick‐and‐mortar” structures, akin to nacre, with interesting combinations of strength, stiffness, and work of fracture, which provide a pathway to making strong and tough lightweight materials.
CrCoNi-based medium- and high-entropy alloys display outstanding damage tolerance, especially at cryogenic temperatures. In this study, we examined the fracture toughness values of the equiatomic ...CrCoNi and CrMnFeCoNi alloys at 20 kelvin (K). We found exceptionally high crack-initiation fracture toughnesses of 262 and 459 megapascal-meters
½
(MPa·m
½
) for CrMnFeCoNi and CrCoNi, respectively; CrCoNi displayed a crack-growth toughness exceeding 540 MPa·m
½
after 2.25 millimeters of stable cracking. Crack-tip deformation structures at 20 K are quite distinct from those at higher temperatures. They involve nucleation and restricted growth of stacking faults, fine nanotwins, and transformed epsilon martensite, with coherent interfaces that can promote both arrest and transmission of dislocations to generate strength and ductility. We believe that these alloys develop fracture resistance through a progressive synergy of deformation mechanisms, dislocation glide, stacking-fault formation, nanotwinning, and phase transformation, which act in concert to prolong strain hardening that simultaneously elevates strength and ductility, leading to exceptional toughness.
Too cold to fracture
Finding structural materials that have good fracture properties at very low temperatures is challenging but is important for fields such as space exploration. Liu
et al
. discovered a high-entropy chromium-cobalt-nickel alloy that has an incredibly high fracture toughness at 20 kelvin (see the Perspective by Zhang and Zhang). This behavior is caused by an unexpected phase transformation that, when combined with other microstructures, prevents crack formation and propagation. The fracture toughness of this alloy makes it potentially useful for a range of cryogenic applications. —BG
CrCoNi-based alloys have very high fracture toughness at 20 kelvin.
The popularization of concentrated solid-solution alloys has prompted a renewed search for atomic-scale chemical order among elements that appear randomly distributed within crystal lattices. ...However, confounding signals in electron diffraction experiments necessitate an immediate reliance on indirect measurements for detecting local order in compositionally complex alloys, as may be interpreted through computer simulations. For instance, both the structure and magnetization of the CrCoNi model system notably contradict ab initio theory for random alloys, but could be reconciled by the widespread presence of chemical short-range order. These simulations additionally find significant magnetic interactions in materials that are often assumed only paramagnetic, motivating further predictions of antiferromagnetism in binary Cr-Ni alloys, which are conventionally understood to have nonmagnetic ground states. This result indicates either the failure of standard theories or, as suggested by anomalous historical measurements, the existence of previously overlooked magnetic order that could persist well above ambient temperatures. In order to realistically model magnetic thermodynamics, a new Monte Carlo approach is developed and found to provide insight into the origin and prevalence of magnetic short-range order.
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